Did you hook the GPS up to the PC after you loaded MM? If so, unload and reload MM. Check thet the GPS setup is correct – MM should auto locate and configure the GPS but settings could have shifted.
Also, is the GPS connecting properly? My Forerunner can say it's charging but not accept a data connection if it's not seated in the cradle properly.
Lastly, if your route has too many waypoints, it might not upload but it will drop out with an error on the GPS display. I convert the route to a track and back to a route again and it seems to work then.
There's lots on MM's website for troubleshooting, too.
Cheers,
Ian